The present invention relates to angular rate sensors and, more particularly, to an angular rate sensor for a vehicle.
Angular rate sensors are useful in vehicle navigation to detect an angular rate of the vehicle body. When the angular rate is integrated, the body angle of the vehicle, with respect to an arbitrary starting body angle, can be calculated.
An angular rate sensor generally relies on a gyro for sensing a change in the body angle of the vehicle on which it is mounted. The inventors have discovered that a drift error of the gyro of an angular rate sensor is temperature sensitive. Due to the temperature sensitivity, if the environmental temperature to which the gyro is exposed changes during use, an unknown drift error is produced. This unknown drift error, which accumulates over time, limits the accuracy of angular rate sensors.
In high-cost systems, a gyro angular rate sensor is mounted in an oven whose temperature is carefully controlled to a fraction of a degree by an automatic temperature control system. The high cost of such a technique forbids its use in low-cost systems such as are required to serve as part of the navigation systems of automobiles and trucks.
An angular rate sensor may be calibrated while the vehicle is stationary by noting the null voltage produced. In Japanese laid-open utility model publication 2-194314, a gyro angular rate sensor calculates a drift compensation factor by noting the null voltage every time the vehicle stops. This drift compensation factor is then used to compensate for drift while the vehicle is moving.
Although the foregoing drift compensation system may be effective to compensate for drift during short episodes of vehicle motion, on a long continuous trip, drift errors grow to serious amounts. In addition, a motor vehicle, and a gyro within it, is exposed to large temperature changes. For example, during cold-weather travel, the temperature in passenger spaces can change by many tens of degrees due to heater operation. In hot-weather travel, the temperature can similarly change due to solar insolation, convective heating and cooling, and air conditioner use. Accordingly, the drift compensation system according to the above utility model suffers serious errors.